1. Field of the Invention
The invention relates in general to solid freeform fabrication and, in particular, to a surface scanning system for generating a plurality of surface height measurements for use in dimensionally normalizing layers of a three-dimensional model being built by a closed loop selective deposition modeling apparatus dispensing a liquid, powder, or paste.
2. Description of the Prior Art
In selective deposition modeling, herein referred to as xe2x80x9cSDMxe2x80x9d, complex parts are produced from a modeling material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques such as SDM incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part.
In SDM, a solid modeling material is physically deposited in successive fashion to form an object. In one type of SDM technology the solid modeling material is extruded as a continuous filament through a resistively heated nozzle. In yet another type of SDM technology, the solid modeling material is jetted or dropped in discrete droplets in order to build up a part. Often, a thermoplastic material having a low-melting point is used as the solid modeling material, which is delivered through a jetting system such as those used in ink-jet printers. One type of SDM process utilizing ink-jet print heads is described, for example, in U.S. Pat. No. 5,555,176 to Menhennett, et al. Another type of SDM process which extrudes a bead of material to build a part is described, for example, in U.S. Pat. No. 5,303,141 to Batchelder et al. Still others dispense two different solidifiable materials such as the SDM process described in U.S. Pat. No. 5,136,515 to Helinski. In addition, other SDM processes dispense sintered powders as described in U.S. Pat. No. 5,017,753 to Deckard, pastes as described in U.S. Pat. No. 6,110,409 to Allanic et al., and liquids as described in U.S. Pat. No. 5,236,637 to Hull.
Although SDM methods have many advantages compared to conventional fabrication methods, they also have inherent problems rooted in the layer by layer building process. One common problem in the layer by layer building process results from the dimensional variability inherent in the building of each layer. These dimensional inaccuracies result from numerous phenomena, such as the accumulated effects of drop volume variation, thermal distortion, errors in deposition, and the like. In addition, the type of geometrical configurations being formed can also influence these inaccuracies, such as the production of web or branching supports. Also, a weakened dispensing jet will contribute to these inaccuracies. If unchecked, these tolerances can accumulate throughout the part as it is built up in height layer by layer. As the thickness of layers are reduced in order to achieve greater surface resolution, the accumulated effects of these undesirable tolerances can substantially distort the resultant part. Thus, most SDM processes require some method to dimensionally normalize or smooth the top working surface of the part while it is being built. In addition, the maximum build rate of the object is limited by the slowest build rate position of a given layer being formed. Generally, most conventional dimensional normalization methods involve systems which physically adjust the vertical height of the part by smoothing or leveling the build material deposited in the layers, often producing waste material. Such systems are typically open loop systems that are hardwired and utilize no active feedback to compensate the build rate of the layer. For example, in one open loop approach, each layer of build material is dispensed at a greater thickness than desired and then the normalizing device is activated to remove the excess build material to achieve the desired thickness. Although distortions between the layers are eliminated, undesirable waste material is generated.
An example of the excess build material approach is found in U.S. Pat. No. 5,943,235 to Earl et al., wherein a pre-heated rotating planarizer is provided to normalize each layer. Under this approach, after a layer of build material has been to deposited by the SDM apparatus in excess of the necessary amount to achieve a desired thickness, the pre-heated cylindrical roller (planarizer) is precisely passed over the deposited material. The rolling planarizer locally melts a portion of the build material of the layer. Some of the material adheres to the surface of the planarizer as it rolls to thereby dimensionally normalize the deposited layer to conform to the desired thickness of the layer. A wiping or scraping device such as a blade is needed to peel or skive off the excess build material from the planarizer, producing waste material. This approach has drawbacks; for instance, the build time is increased due to the deposition of material that is later removed by the planarizer as waste. In addition, the rolling planarizer must be manufactured to precise tolerances in order to achieve the desired accuracy. It is also difficult to precisely regulate and maintain the temperature of the surface of the planarizer. The planarizer is also thermally inefficient as it consumes a significant amount of energy that is undesirably dissipated into the environment. The planarizer also occupies a significant amount of space within the SDM apparatus and thereby limits the over-travel distance of the dispensing carriage. The planarizer also has moving parts that are subject to wear and degradation. Airborne contaminants are also prone to accumulate on the planarizer. In short, the heated rotating planarizer adds significant cost and complexity to an SDM apparatus, occupies precious space, adds inertia, is subject to wear, requires maintenance and adjustment, and increases build time.
Other approaches to providing a system to dimensionally normalize a part while being built by an SDM apparatus are found in U.S. Pat. No. 5,859,775 to Barlage, III et al. and U.S. Pat. No. 5,572,431 to Brown et al. Under these approaches, a heated body is selectively driven across the dispensed build material in response to a sensed deviation in order to melt and displace the build material. The excess material is then sucked off by a vacuum source connected to the heated body. Thus, dimensional normalization is discretely accomplished by expelling waste material, thereby increasing build time.
Undesirably, all these approaches involve making physical contact with the layer of the object as it is built. Because of this, the types of material that can be dispensed to form the layers are limited to those that can both be locally melted and removed, or can be planed or machined. Furthermore, they must leave a desirable surface finish after being normalized. In addition, removal of waste material generally runs contrary to the advantages of the additive nature of the SDM fabrication process.
Thus, there is a need to provide an SDM process capable of dimensionally normalizing layers of a three-dimensional object without generating waste build material. There is also a need to decrease build time by eliminating the steps of depositing excess build material for each layer. There is also a need to dimensionally normalize the layers without making physical contact with the layers. These and other difficulties of the prior art have been overcome according to the present invention.
The present invention provides its benefits across a broad spectrum of three-dimensional building processes. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As will be understood, the basic apparatus and methods taught herein can be readily adapted to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
It is one aspect of the present invention to form a three-dimensional object with an SDM apparatus at the highest build speeds possible.
It is another aspect of the present invention to dimensionally normalize the layers of a three-dimensional object by an SDM apparatus without generating waste material.
It is yet another aspect of the present invention to dimensionally normalize the layers of a three-dimensional object being formed by an SDM apparatus without making physically contact with the build material already dispensed in the layers with a separate device.
It is still yet another aspect of the present invention to form a three-dimensional object according to a closed loop SDM technique responsive to feedback data.
It is a feature of the present invention to actively monitor the surface height of any portion of a layer of a three-dimensional object while being formed by an SDM apparatus.
It is another feature of the present invention to develop height data signals of the surface of a portion of a layer of a three-dimensional object formed by an SDM apparatus.
It is yet another feature of the present invention to process the height data signals to produce feedback data for selectively dispensing build material in desired locations on the surface of the portion of a layer.
It is yet another feature of the present invention to utilize a surface scanning system consisting of an energy beam directed onto the surface of the object to establish an illumination zone that emits scattered light, and a detector having sensors which monitor the surface height condition of the object by collecting the scattered light.
It is an advantage of the present invention that each layer of an object formed by the SDM apparatus can be dimensionally normalized without generating waste material and thereby eliminate the need for a waste collection tank and waste umbilical.
It is another advantage of the present invention that the build speed of the SDM apparatus is significantly increased by eliminating the time involved in depositing build material in excess for each layer that must be removed as waste.
It is yet another advantage of the present invention that maintenance and service of the SDM apparatus can be significantly decreased as a result of the elimination of mechanical systems previously needed to physically contact and remove excess build material.
It is still yet another advantage of the present invention that a closed loop system is used that can actively adjust the build process in response to monitored imperfections on the surface of the layers during the build process.
And it is still yet another advantage of the present invention that a wider variety of materials can be dispensed from an SDM apparatus to form three-dimensional objects.
These and other aspects, features, and advantages are achieved/attained in the method and apparatus of the present invention that employs a surface scanning system that generates a plurality of surface height data signals of a layer of build material dispensed in response to data descriptive of the layer of a three-dimensional object being built by a SDM technique. The height data signals are processed to establish feedback data that is then utilized to selectively dispense the build material in desired locations on the layer such as in low areas to achieve a desired thickness, and to avoid dispensing material in locations that may have excess material.